Historically, the telephone, which comes from the Greek word ‘tele’, meaning from afar, and ‘phone’, meaning voice or voice sound, is said to have been invented on Mar. 10, 1876 in Boston, Mass. by Alexander Graham Bell. The principle of the telephone was conceived as early as 1874 combining electricity and voice which led to Bell's actual invention of the telephone in 1876.
U.S. Pat. No. 174,465 issued Mar. 3, 1876 for improvements in telegraphy is now considered to be the most valuable patent ever issued.
Telstar, the world's first international communications satellite, years later was placed into orbit on Jul. 10, 1962 in a collaboration between NASA and the Bell System. Today satellites in geosynchronous orbit are used mostly for long distance service.
The basic concept of cellular phones which began in 1947 with crude mobile car phones resulted in the realization that using small cells or range of service area with frequency re-use could increase the traffic capacity of mobile phones substantially. However, at this point in time the technology was nonexistent. The cellular telephone is in fact a type of two-way radio which in 1947 AT&T proposed at the FCC allocated large number of radio spectrum frequencies so that widespread mobile phone service could become feasible and provide AT&T an incentive to research the new technology. The FCC's decision to limit the cellular phone frequencies in 1947 resulted in the possibility of only 23 cellular phone conversations which could occur simultaneously in the same service area. In 1968 this was increased. Thereafter, a cellular phone system was proposed by Bell Laboratories. In 1977 AT&T Bell Labs constructed and operated a prototype cellular phone system. In 1981 Motorola and America Radio Phone started a second U.S. cellular radio phone system test in the Washington/Baltimore area. Suddenly consumer demand quickly outstripped the cellular phone system's 1982 standards so that by 1987 cellular phone subscribers exceeded one million and the airwaves were crowded. To stimulate the growth of new cellular phone technology, the FCC declared in 1987 that cellular phone licenses may employ alternative cellular phone technologies in the 800 megahertz band.
Digital wireless and cellular find their roots back in the 1940s when commercial mobile telephony began. On Jun. 17, 1946 in St. Louis, Mo., AT&T and Southwestern Bell introduced the first American commercial mobile radio telephone service and mobile telephony a channel is a pair of frequencies, one frequency to transmit on and one to receive.
A cell phone is a portable telephone which receives or sends messages through a cell site or transmitting tower. Radio waves are used to transfer signals to and from the cell phone, each cell site having a range of 3-5 miles and overlapping other cell sites. All of the cell sites are connected to one or more cellular switching exchanges which can detect the strength of the signal received from the telephone. As the telephone user moves or roams from one cell area to another, the exchange automatically switches the call to the cell site with the strongest signal. The term ‘cell phone’ is uncommon outside the United States and Japan. However, almost all mobile phones use cellular technology including GSM, CDMA and the old analog mobile phone systems. Hence, the term ‘cell phone’ has been regarded by many to designate any mobile telephone system. An exception to mobile phones which employ cellular technology are satellite phones; for example, the Iridium phone system which is very much like a cell phone system except the cell sites are in orbit. Marine radio telephone satellites administered by Inmarsat have a completely different system. The Inmarsat satellite system simply retransmits whatever signals it receives with a mobile station's actually logging into a ground station.
With the advent of the Globalstar® satellite telephone system, a great advance in the art was recognized by virtue of a basic telephonic satellite technology which provided a constellation of 48 satellites in low earth orbit which were much simpler to build and less expensive than those of Iridium employing a radically different technology which employs code division multiple access, or CDMA, technology, converting speech signals into a digital format and then transmitting it from the Globalstar® satellite phone up to satellite systems and down to the ground station. Every call on the Globalstar® system possesses its own unique code which distinguishes it from the other calls sharing the airwaves at the same time, and employing CDMA provides signals which are free of interference, cross-talk or static. CDMA was introduced in 1995 and soon became the fastest growing wireless technology and one that was chosen by Globalstar® for use in its satellite communications network, which service Globalstar® launched in 2000.
The key features of the Globalstar® satellite phone employing CDMA provide unique forward and reverse links, direct sequence spread spectrum, seamless soft handoff, universal frequency re-use, propagation through multiple overlapping beams on multiple satellites for diversity, and variable rate transmission.
The Globalstar® satellite phone service is delivered through 48 low earth orbiting satellites providing both voice and data services. The so-called Globalstar® LEO constellation consists of satellites arranged in a Walker constellation, and each satellite is approximately 700 miles from the earth which allows for the highest quality voice clarity of any satellite phone in the industry. At the heart of the Globalstar® system as initially proposed is Qualcomm's adaptation of code division multiple access technology which provides Globalstar's® digital satellite service, resulting in a technology which provides signal security, superior quality, fewer dropped calls and greater reliability. Calls can be made from any gateway via any satellite of the system to any user terminal, as long as the satellite is co-visible from both gateway and user terminal. This co-visibility is what defines a gateway service area; at least 24 gateways around the globe are used to provide worldwide coverage. Each satellite serves at least 2,000 simultaneous users.
The Globalstar® system employs redundancy with every call that a customer places so that a call is routed through as many as four satellites which then combine the signal into a single static-free call. In the event that one of the paths to one of the satellites is blocked, the other satellites keep the call from terminating, applying the technology of path diversity which minimizes dropped calls and enhances the quality of the Globalstar® satellite phone service. The Globalstar® system employs bent pipe technology which allows a call to be first beamed up to the satellite and then retransmitted to a relatively close gateway. The call is then sent through its call destination through land line or cellular networks. The Globalstar® gateway carries out all the processing and switching of the calls which improves the reliability of the call delivery, unlike the Iridium system which requires satellite-to-satellite transmission.
In addition, the Globalstar® system, which provides reliable call delivery with voice characteristics the same or better than conventional telephony, complements the current cellular telephone systems in existence by allowing the user to first use conventional cellular, which is far less expensive but totally dependent upon the proximity of cell sites for its reliability, and then allows the user to select the Globalstar® satellite system where cell sites are far too distant to be reliable or in remote locations where these sites are non-existent. Code division multiple access, which refers to a multiple access scheme where stations use spread spectrum modulations and orthogonal codes to avoid interfering with one another, is typically employed in Globalstar® systems. The CDMA modulation technique is one of several techniques for facilitating communications in which a large number of system users are present. Other multiple access communications system techniques such as time division multiple access (TDMA), frequency division multiple access (FDMA), and AM modulation schemes such as amplitude expanded single sideband (ACSSB) are known in the art. The spread spectrum modulation technique of CDMA is found to have significant advantages over these modulation techniques for multiple access communications systems. CDMA techniques in multiple access communications systems are disclosed in U.S. Pat. No. 4,901,307 entitled Spread Spectrum Multiple Access Communication System Using Satellite or Terrestrial Repeaters, the disclosure thereof is incorporated by reference.
In this patent, a multiple access technique is disclosed where a large number of mobile telephone system users each having a transceiver communicate through satellite repeaters or terrestrial base stations, also referred to as cell sites stations, cell sites, or for short cells, using code division multiple access (CDMA) spread spectrum communication signals. Frequency spectrum employed in CDMA can be reused multiple times, thus permitting an increase in system user capacity. The CDMA is found to result in a much higher spectral efficiency than can be achieved using other multiple access techniques.
Satellite channels employing this system typically experience fading that is characterized as Rician. Accordingly, this signal is found to consist of a direct component summed with a multiple reflected component having a Rayleigh fading statistic. A power ratio between the direct and reflected component is typically found to be on the order of 6 to 10 dBs depending upon the characteristics of the mobile unit antenna and the environment about the mobile unit. Contrasted to the satellite channel, the terrestrial channel experiences signal fading that typically consists of the Rayleigh faded component without a direct component. This terrestrial channel is found to present a more severe fading environment than the satellite channel in which the Rician fading is the dominant fading characteristic.
The Rayleigh fading characteristics experienced in the terrestrial signal is found to be caused by the signal being reflected from many different features of the physical environment, resulting in a signal which arrives at a mobile unit receiver from many directions with different transmission delays. In the UHF frequency bands which are usually employed for mobile radio communications, including cellular mobile telephone systems, there is found to be significant phase differences in signals traveling on different paths which provides the possibility of destructive summation of the signals causing occasional deep fades. Physical position of the mobile unit is a strong function of the terrestrial channel fading so that small changes in the position of the mobile unit change the physical delays of all the signal propagation paths which further result in a different phase for each path. The motion of the mobile unit through the environment can result in a rapid fading process; for example, employing 850 MHz cellular radio frequency band, the fading can typically be as fast as one fade per second per mile per hour of the vehicle speed. This level of fading is found to be extremely disruptive to signals in a terrestrial channel, resulting in poor communication quality. Quality may be improved by providing additional power to overcome the fading, which in itself affects both the user in excessive power consumption and the system by increased interference. Certain CDMA modulation techniques disclosed in U.S. Pat. No. 4,901,307 offer some advantages over narrow band modulation techniques using communication systems employing satellite or terrestrial repeaters. The terrestrial channel is found to pose special problems to any communication system, particularly with respect to multiple path. These problems may be overcome by using CDMA techniques which overcome the special problems of the terrestrial channel by mitigating the adverse effect of multipath, for example fading, while also exploiting the advantages of multipath.
CDMA cellular telephone systems allow the same frequency band to be employed for communication in all calls. CDMA waveform properties that provide processing gain are also used to discriminate between signals that occupy the same frequency band. Furthermore, the high speed pseudo-noise PN modulation allows many different propagation paths to be separated provided the difference in path delay exceed the PN chip duration; i.e., 1/bandwidth. It is found that if a PN chip rate of approximately one MHz is employed in a CDM system, the full spread spectrum processing gain equal to the ratio of the spread bandwidth to system data rate can be employed against paths that differ by more than one microsecond in path delay from desired path. It is found that a one microsecond path delay differential corresponds to differential path distance of approximately 1,000 feet, the urban environment typically providing differential path delays in excess of one microsecond and up to 10-20 microseconds in some areas. When narrow band modulation systems are employed, such as at analog FM modulation, by conventional telephone systems, the existence of multiple paths results in severe multipath fading. By employing wideband CDMA modulation, the different paths may be discriminated against in the demodulation process which greatly reduces the severity of multipath fading. Although multipath fading is not totally eliminated using CDMA discrimination techniques, there will occasionally exist paths with delayed differentials of less than the PN chip duration for the particular system. For signals which possess path delays on this order, it is found that signals cannot be discriminated against in the demodulator, resulting in some degree of fading.
It becomes apparent that some form of diversity is desirable which would permit a system to reduce fading. One such system is diversity which mitigates the deleterious effects of fading. The three major types of diversity which may be employed are time diversity, frequency diversity and space diversity. Time diversity is found to be best obtained by the use of repetition, time interleaving and error detection and coding which is a form of repetition.
CDMA by its inherent nature possessing a wide band signal which offers a form of frequency diversity by spreading the signal energy over a wide bandwidth, resulting in a small part of the CDMA signal bandwidth experiencing selective fading effects.
Space or path diversity is obtained by providing multiple signal paths through simultaneously links from a mobile user through two or more cell sites. Path diversity may be obtained by exploiting the multipath environment through spread spectrum processing by allowing a signal arriving with different propagation delays to be received and processed separately. In U.S. Pat. No. 5,101,501 entitled Soft Handoff in a CDMA Cellular Telephone System, and U.S. Pat. No. 5,109,390 entitled Diversity Receiver in a CDMA Cellular Telephone System, examples of path diversity are illustrated. Further control of deleterious effects in a CDMA system may be realized by controlling transmitter power. Such a system for cell site mobile unit power control is disclosed in U.S. Pat. No. 5,056,109 entitled Method and Apparatus for Controlling Transmission Power in a CDMA Cellular Mobile Telephone System. Techniques as disclosed in U.S. Pat. No. 4,901,307 contemplate the use of coherent modulation and demodulation for both directions of the link in mobile satellite communications. A pilot carrier signal as a coherent phase reference for the satellite to mobile link and the cell to mobile link is disclosed. It is found, however, that the severity of multipath fading experienced in the terrestrial cellular environment with the resulting phase disruption of the channel precludes usage of coherent demodulation techniques for the mobile to cell link.
Relatively long PN sequences with each user channel being assigned a different PN sequence are also disclosed in U.S. Pat. No. 4,901,307. The different user signals may be discriminated upon reception employing the cross correlation between different PN sequences and the auto correlation of a PN sequence for all time shifts other than zero where both have a zero average value. Although the cross correlations average zero for a short time interval, such as an information bit time, the cross correlation follows a binomial distribution since PN signals are not orthogonal. As such, signals interfere with each other much the same as if they were wide bandwidth Gaussian noise resulting in other user signals or mutual interference noise ultimately limiting the achievable capacity.
Multipath can provide path diversity to a wide band PN CDMA system which uses greater than 1 MHz bandwidth if two or more paths are available with greater than one microsecond differential path delay. Two or more PN receivers can be employed to separately receive these signals. These signals typically will exhibit independence in multipath fading, i.e., they usually do not fade together, the outputs of the two receivers can be diversity combined. It is found that a loss in performance in this situation only occurs when both receivers experience fades at the same time, hence two or more PN receivers in combination with a diversity combiner may be employed utilizing a waveform that permits path diversity combining operations to be performed.
In U.S. Pat. No. 4,901,307 filed Oct. 17, 1986, issued Feb. 13, 1990, a communication system which accommodates a large number of users throughout a variety of user environments from high density urban to very low density rural is provided which results in a multiple access communication system having high simultaneous user capacity.
In U.S. Pat. No. 5,101,501 filed Nov. 7, 1989, issued Mar. 31, 1992, there is disclosed a CDMA cellular telephone system wherein the same frequency band is used for all cells employing CDMA waveform properties that provide processing gains which are also used to discriminate between signals that occupy the same frequency band.
In U.S. Pat. No. 5,103,459 filed Jun. 25, 1990, issued Apr. 7, 1992, there is disclosed spread spectrum communication techniques, particularly CDMA techniques, in the mobile cellular telephone environment which provide features to vastly enhance system reliability and capacity over other communication system techniques overcoming fading and interference while providing greater frequency reuse and enabling a substantial increase in the number of system users.
In U.S. Pat. No. 5,109,390 filed Nov. 7, 1989, issued Apr. 28, 1992, there is disclosed a CDMA cellular telephone system where the same frequency band is used for communication in all cells to provide a cellular telephone system in which a receiver design facilitates reception and processing of the strongest signals transmitted from one or more cell sites, the signals being multipath signals from a single cell site or signals transmitted by multiple cell sites.
In U.S. Pat. No. 5,233,626 filed May 11, 1992, issued Aug. 3, 1993, there is disclosed a repeater diversity spread spectrum communication system providing substantially fade free communications between a transmitter (1) and a receiver (7). A transmitted signal is relayed through a plurality of linear communications repeaters (3-6) that produce copies of the transmitted signal, the copies each arriving through an independently fading signal path. The receiver processes the received signal copies to equalize them to one another in delay, frequency, and phase, and then combines the multiple received and equalized signal copies to produce a composite signal having a greatly reduced fading depth.
In U.S. Pat. No. 5,267,261 filed Mar. 5, 1992, issued Nov. 30, 1993, there is provided a system for directing handoff in mobile station communication between base stations which employ code division multiple access techniques.
In U.S. Pat. No. 5,267,262 filed Oct. 8, 1991, issued Nov. 30, 1993, there is disclosed a CDMA cellular mobile telephone wherein the transmitter power of the mobile units are controlled so as to produce at the cell site a nominal received signal power from each and every mobile unit transmitter operating within the cell. Thus, the transmitter power is controlled in the terrestrial channel and the cell diversity environment so as to overcome deleterious fading without causing unnecessary system interference.
In U.S. Pat. No. 5,303,286 filed Mar. 29, 1991, issued Apr. 12, 1994, there is disclosed a radio communication system capable of servicing a roaming user or the like outside the range of terrestrial relay stations including a packet switched network and database of roaming users, a satellite communications system having at least one, but usually a plurality of orbiting satellites over a terrestrial satellite service area, a satellite control center and a plurality of terrestrial communication links wherein call setup is controlled by processors and databases onboard the orbiting satellites and wherein only after the satellite link for the communication channels is completed, does control and switching rely on ground base system such that the orbiting satellites are integrated into a ground based telephone network and tariff structure.
In U.S. Pat. No. 5,309,474 filed Mar. 27, 1992, issued May 3, 1994, there is disclosed spread spectrum communication techniques, particularly CDMA, in a mobile cellular telephone environment which provides features to vastly enhance system reliability and capacity over other communication system techniques.
In U.S. Pat. No. 5,416,797 filed Jan. 24, 1992, issued May 16, 1995, there is disclosed a system for constructing PN sequences that provide orthogonality between the users so that mutual interference will be reduced allowing higher capacity and better link performance, employing spread spectrum communication techniques, particularly CDMA, in a mobile cellular telephone environment.
In U.S. Pat. No. 5,715,297 filed Sep. 15, 1995, issued Feb. 3, 1998, there is disclosed a radio communication system capable of servicing a roaming user or the like outside the range of terrestrial relay stations which includes a packet switched network and database of roaming users, a satellite communications system having at least one, but usually a plurality of orbiting satellites over a terrestrial satellite service area, a satellite control center and a plurality of terrestrial communication links, wherein call setup is controlled by processors and databases onboard the orbiting satellites and wherein only after the satellite link for the communication channels is completed, does control and switching rely on ground based equipment such that the orbiting satellites are integrated to a ground based telephone network and tariff structure.
In U.S. Pat. No. 5,903,837 filed Sep. 22, 1997, issued May 11, 1999, there is disclosed a radio communication system capable of servicing a roaming user or the like outside the range of terrestrial relay stations which includes a packet switched network and database of roaming users, a satellite communications system having at least one, but usually a plurality of orbiting satellites over a terrestrial satellite service area, a satellite control center and a plurality of terrestrial communication links wherein call setup is controlled by processors and databases onboard the orbiting satellites and wherein only after the satellite link for the communication channel is completed, does control and switching rely on ground based equipment such that the orbiting satellites are integrated into a ground based telephone network and tariff structure.
In U.S. Pat. No. 6,072,768 filed Sep. 4, 1996, issued Jun. 6, 2000, there is disclosed a communication system having a satellite communication component comprising at least one satellite and at least one terrestrial gateway and also a wireless terrestrial communication component comprising at least one repeater and at least one mobile switching center, the gateway and switching center coupled together by a first mobile applications part network, the gateway and the mobile switching center further coupled to a terrestrial communication network, further including at least one dual mode or higher tri-mode user terminal comprising a first transceiver for bidirectionally communicating with the gateway through the satellite, a second transceiver for bidirectionally communicating with the mobile switching center through the repeater and a controller responsive to one of a user selected or a gateway selected protocol for selectively enabling either the first or the second transceiver for conveying a user communication to a terrestrial communication network.
In U.S. Pat. No. 6,529,485 filed Oct. 20, 1998, issued Mar. 4, 2003, there is disclosed a method for generating a Doppler-free local clock in a communications network having a master reference terminal (400) and a terminal (200) exchanging references and management bursts, includes steps for determining a transmit timing correction value responsive to the management burst received by the master reference terminal (400), determining a receive timing correction value responsive to the reference burst received by the terminal (200), and adjusting the frequency of a clock responsive to both the transmit timing correction value and the receive timing correction value to thereby generate the Doppler-free local clock.
In U.S. Pat. No. 6,640,236 filed Aug. 31, 1999, issued Oct. 28, 2003, there disclosed an apparatus for generating a PN sequence with an arbitrary number of bits where the number of bits is provided in parallel with each clock pulse, allowing the sequences to be generated at high speed when needed and allowing parallel processing in the acquisition and demodulation processes.
In U.S. Pat. No. 6,693,951 filed Jul. 23, 1999, issued Feb. 17, 2004, there is disclosed implementation of spread spectrum communication techniques, particularly CDMA, in a mobile cellular telephone environment which provides features that vastly enhance system reliability and capacity over other communication system techniques, overcoming, for example, fading and interference while promoting greater frequency reuse, enabling a substantial increase in the number of system users.
In U.S. Pat. No. 6,714,780 filed Jun. 12, 2001, issued Mar. 30, 2004, there is disclosed a multibeam communication system having a user terminal, a communications station for transmitting information to and receiving information from the user terminal and a plurality of beam sources where each beam source projects a plurality of beams and where a communication link between the user terminal and the communications station is established on one or more beams, providing a system and method for reducing call dropping rates while maintaining a desired level of beam source diversity.
In U.S. Pat. No. 6,813,259 filed Jul. 15, 1998, issued Nov. 2, 2004, there is disclosed a method and apparatus for providing a low 2-point Cell delay variation (CDV) for cell or packet transmissions via a TDMA or TDM network, where the cells or packets are assembled in bursts or slots for transmission. In order to permit a TDMA or TDM network that carries cells or packets between source and destination pairs to guarantee that a desired 2-point CDV will be met, for example, a 3 ms CDV required for Class 1 traffic, each cell is associated with a transmitted TDMA or TDM frame. Using a time counter and a frame counter in a transmitter interface, the cell or packet has appended to it a time count and a frame count that is sent across the network and made available to the receiving TDMA/TDM terminal. The receiving terminal uses this timing information to perform traffic shaping of the cell or packet stream, thereby reducing the impact of the 2-point CDV as well as the effect of cell clumping prior to distribution on a terrestrial network.
In U.S. Pat. No. 6,839,007 filed Sep. 9, 2002, issued Jan. 4, 2005, there is disclosed embodiments which address the need for reliable transmission of higher priority data within a frame wherein an inner code is applied to one or more partial segments of a transmitted data frame, in addition to an outer code applied to the entire frame, the inner code segment being retained when the inner decoding decodes without error providing the benefit of reducing the number of retransmissions of higher priority data, as well as reducing delay for time sensitive segments of the frame.
Various satellite telephone systems have been proposed, including those as depicted in the FCC filing for “Authority to Launch and Operate a Satellite System to Provide Mobile Satellite Services in the 2 GHz Bands” dated Nov. 3, 2000, relating to the Globalstar® system, which is hereby incorporated by reference; the FCC filing in the matter of Mobile Satellite Ventures Subsidiary, LLC for “Minor Amendment of Application to Launch and Operate a Replacement L Band Mobile Service Satellite at 101° West” dated Nov. 18, 2003; and the FCC filing by Thoraya which depicts a one GEO satellite system to provide a satellite telephone service.
Thus, it can be seen from the inception of the telephone through its various phases of improvement, cellular to satellite cellular telephony, a vast number of advances have been made which provide a modern, efficient and affordable telephone system which today, in many cases, supplants the existing telephone system and may in the future do so on an increasing basis.
There is, however, a continuing demonstrated need to provide improved satellite constellation systems, preferably LEO systems, which provide multiple beams to a plurality of users and employ at least one gateway connected to a PSTN communicating with a user over the constellation where each of the users within a given frequency band is distinguished from another employing orthogonal codes.
Although previous patents such as U.S. Pat. No. 4,901,307 describe or reference a multi-beam satellite system, these beams are considered to cover fixed regions on the ground, which requires a GEO satellite. In this case, the same sort of hand-off of a user terminal from beam to beam can be used as is used in a terrestrial cellular system. However, the '307 patent does not address the case where the beams and satellites are rapidly moving as they are in a MEO or LEO system, since it was written in an era that preceded the satellite technology that enabled large numbers of relatively smaller satellites (such as Globalstar's®) to be economically and reliably launched and controlled. Therefore, the hand-off issues described in the '307 patent are much simpler than those encountered in the Globalstar® system or similar LEO or MEO systems, or even those encountered in GEO systems which have dynamically varying beam shapes, which is another technological advance that is now feasible in satellite systems. That patent also does not address packet data services, since those were not widely used in the time frame of the patent. Other patents that address packet data services also do not address the LEO, MEO or dynamic beam GEO systems. The present invention describes a multi-beam LEO, MEO or GEO satellite system that can be used to provide packet data services (in addition to voice) for mobile users, that can be either initiating or receiving packet data calls over the system, while communicating with either a fixed or mobile user anywhere in the world.